The present invention relates to magnetic systems, and more particularly to a method and system for providing spin-dependent tunneling sensors suitable for use as cells in a magnetic memory.
Because of their high magnetoresistance ratio, spin dependent tunneling sensors are currently of interest for use in a variety of devices, including magnetic memories such as magnetic random access memories (MRAM). FIG. 1 depicts a conventional bottom spin-dependent tunneling sensor 10. The conventional spin-dependent tunneling sensor 10 includes a seed layer 12, an antiferromagnetic layer 14, a pinned layer 16, a tunneling barrier 18, a free layer 20 and a capping layer 22. The pinned layer 16 and the free layer 20 are ferromagnetic. The antiferromagnetic layer 14 fixes, or pins, the magnetization of the pinned layer 16 in a particular direction. The magnetization of the free layer 20 is free to rotate in response to a writing field provided at the conventional spin-dependent tunneling sensor 10. The tunneling barrier 18 is an insulator such as alumina and is thin enough to allow charge carriers to tunnel between the free layer 20 and the pinned layer 16. Based on the orientation of the magnetizations of the free layer 20 and the pinned layer 16, the resistance and thus the current through the conventional spin-dependent tunneling sensor 10 changes.
FIG. 2 depicts another conventional spin-dependent tunneling sensor 30. The conventional spin-dependent tunneling sensor 30 includes a seed layer 32, an antiferromagnetic layer 34, a conventional synthetic pinned layer 36, a tunneling barrier 44, a free layer 46 and a capping layer 48. The conventional synthetic pinned layer 36 includes two ferromagnetic layers 38 and 42 separated by a nonmagnetic spacer layer 40. The conventional spin-dependent tunneling sensor 30 functions similarly to the conventional spin-dependent tunneling sensor depicted in FIG. 1.
Although the conventional spin-dependent tunneling sensors 10 and 30 function, one of ordinary skill in the art will readily recognize that the conventional spin-dependent tunneling sensors 10 and 30 do not have a symmetric response to an external magnetic field. FIG. 3 depicts a hysteresis loop 50 of the conventional spin-dependent tunneling sensor 10 or 30. The hysteresis loop 50 indicates the magnetization of the conventional spin-dependent tunneling sensor 10 or 30 versus external field applied to the conventional spin-dependent tunneling sensor 10 or 30. The hysteresis loop 50 is shifted from being symmetric about a zero external field. This occurs because of an interlayer coupling (known as orange peel coupling) between the free layers 20 and 46 and the pinned layers 16 and 36, respectively. The tunneling barriers 18 and 44 have a relatively rough upper surface because they have an antiferromagnetic layer 14 beneath them. Thus, the free layers 20 and 46 also have rough surfaces. The rough interfaces of the free layers 20 and 46 and the conventional pinned layers 16 and 36 result in a high interlayer coupling. The conventional spin-dependent tunneling sensors 10 and 30 thus behave as though there is an additional field applied to the free layers 20 and 46. The response of the conventional spin-dependent tunneling sensors 10 and 30 are thus shifted from being symmetric about a zero external field. Because the magnetization of the conventional spin-dependent tunneling sensors 10 and 30 are asymmetric with respect to an external applied field, the magnetoresistance of the conventional spin-dependent tunneling sensors 10 and 30 is also asymmetric.
FIG. 4 depicts a conventional top pinned spin-dependent tunneling sensor 60. The asymmetry of the conventional spin-dependent tunneling sensors 10 and 30 can be addressed using a conventional top pinned spin-dependent tunneling sensor 60. The conventional top pinned spin-dependent tunneling sensor 60 includes a seed layer 62, a free layer 64, a tunneling barrier 66, a conventional synthetic pinned layer 68, an antiferromagnetic layer 76 and a capping layer 78. The conventional synthetic pinned layer includes two ferromagnetic layers 70 and 74 separated by a thin, nonmagnetic spacer layer 72. The free layer 64 and conventional synthetic pinned layer 68 operate in a manner that is analogous to the conventional spin-dependent tunneling sensors 10 and 30.
Because the free layer 64 is not formed on the thick layer including a tunneling barrier, a pinned layer and an antiferromagnetic layer, the conventional spin-dependent tunneling device 60 is not subject to a high interlayer coupling. However, the conventional spin-dependent tunneling device 60 has a poorly pinned conventional synthetic pinned layer 68. The conventional synthetic pinned layer 68 and the antiferromagnetic layer 76 are grown above the tunneling barrier 66, which is amorphous. As a result, the antiferromagnetic layer 76 may be of poor quality. Consequently, the exchange coupling between the antiferromagnetic layer 76 and the conventional synthetic pinned layer 68 is reduced. As a result, the magnetization of the conventional synthetic pinned layer 68 is poorly pinned and may move in response to a writing field. Thus, the signal from the conventional spin-dependent tunneling device 60 may be unreliable.
Accordingly, what is needed is a system and method for providing an improved spin-dependent tunneling sensor. The present invention addresses such a need.
The present invention provides a method and system for providing a top pinned spin-dependent tunneling sensor. The method and system comprise providing a free layer, a tunneling barrier, a synthetic pinned layer and an antiferromagnetic layer. The free layer is ferromagnetic. The tunneling barrier is an insulator. The tunneling barrier is disposed between the free layer and the synthetic pinned layer. The synthetic pinned layer is ferromagnetic and includes a ferromagnetic top layer. The synthetic pinned layer is between the tunneling barrier and the antiferromagnetic layer. The ferromagnetic top layer acts as a seed layer for the antiferromagnetic layer.
According to the system and method disclosed herein, the present invention provides a top pinned spin-dependent tunneling sensor that has improved pinning, between the synthetic pinned layer and the antiferromagnetic layer.